On Mon, Nov 24, 2025 at 10:21 PM Victor Do Nascimento
<[email protected]> wrote:
>
> The iteration count profitability check is irrelevant for uncounted
> loops given that, even at runtime, the number of iterations is unknown
> at the start of loop execution.
>
> Likewise, the test for skipping the vectorized version of the loop is
> based on whether the number of iterations that will be run for the
> loop and whether it is smaller than the vectorization factor. As this
> is undetermined, the check can never be run for uncounted loops.
>
> Consequently, we skip these checks.
Apart from the two other comments I made the series looks OK overall.
Please work with Tamar to get those bits sorted out.
On this patch I am now raising the cost model question - you skip any
computed profitability check in this patch, but shouldn't you instead
have made sure profitability is always there, thus no check is needed
in the first place? I'd have expected asserts that we do not run into
these code paths with uncounted loops.
How is the approach to avoid turning random uncounted loops into
vector loops and thus regressing performance a lot?
Did you perform any benchmark runs with the patch set in?
Thanks,
Richard.
> gcc/ChangeLog:
>
> * tree-vect-loop-manip.cc (vect_loop_versioning): skip
> profitability check for uncounted loops.
> * tree-vect-loop-manip.cc (vect_do_peeling): Disable vector
> loop skip checking.
> ---
> gcc/tree-vect-loop-manip.cc | 17 ++++++++++-------
> 1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> index 3caaba30897..6bc474a3de0 100644
> --- a/gcc/tree-vect-loop-manip.cc
> +++ b/gcc/tree-vect-loop-manip.cc
> @@ -3289,11 +3289,13 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree
> niters, tree nitersm1,
> because we have asserted that there are enough scalar iterations to
> enter
> the main loop, so this skip is not necessary. When we are versioning
> then
> we only add such a skip if we have chosen to vectorize the epilogue. */
> - bool skip_vector = (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> - ? maybe_lt (LOOP_VINFO_INT_NITERS (loop_vinfo),
> - bound_prolog + bound_epilog)
> - : (!LOOP_VINFO_USE_VERSIONING_WITHOUT_PEELING
> (loop_vinfo)
> - || vect_epilogues));
> + bool skip_vector = false;
> + if (!uncounted_p)
> + skip_vector = (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> + ? maybe_lt (LOOP_VINFO_INT_NITERS (loop_vinfo),
> + bound_prolog + bound_epilog)
> + : (!LOOP_VINFO_USE_VERSIONING_WITHOUT_PEELING (loop_vinfo)
> + || vect_epilogues));
>
> /* Epilog loop must be executed if the number of iterations for epilog
> loop is known at compile time, otherwise we need to add a check at
> @@ -4152,13 +4154,14 @@ vect_loop_versioning (loop_vec_info loop_vinfo,
> tree version_simd_if_cond
> = LOOP_REQUIRES_VERSIONING_FOR_SIMD_IF_COND (loop_vinfo);
> unsigned th = LOOP_VINFO_COST_MODEL_THRESHOLD (loop_vinfo);
> + bool uncounted_p = LOOP_VINFO_NITERS_UNCOUNTED_P (loop_vinfo);
>
> - if (vect_apply_runtime_profitability_check_p (loop_vinfo)
> + if (!uncounted_p && vect_apply_runtime_profitability_check_p (loop_vinfo)
> && !ordered_p (th, versioning_threshold))
> cond_expr = fold_build2 (GE_EXPR, boolean_type_node, scalar_loop_iters,
> build_int_cst (TREE_TYPE (scalar_loop_iters),
> th - 1));
> - if (maybe_ne (versioning_threshold, 0U))
> + if (!uncounted_p && maybe_ne (versioning_threshold, 0U))
> {
> tree expr = fold_build2 (GE_EXPR, boolean_type_node, scalar_loop_iters,
> build_int_cst (TREE_TYPE (scalar_loop_iters),
> --
> 2.43.0
>
